Question

Among the following cations, the number of cations which will give characteristic precipitate in their identification tests with
\(K_4\)[Fe(CN)\(_6\)] is : \[ {Cu}^{2+}, \, {Fe}^{3+}, \, {Ba}^{2+}, \, {Ca}^{2+}, \, {NH}_4^+, \, {Mg}^{2+}, \, {Zn}^{2+} \]

Hint:

In tests with \( {K}_4[{Fe(CN)}_6] \), only certain cations like Cu²⁺, Fe³⁺, and Zn²⁺ give characteristic precipitates. Cations such as Ba²⁺, Ca²⁺, NH₄⁺, and Mg²⁺ typically do not react in this way.

Answer 1

Precipitates with \( K_4[{Fe(CN)}_6] \) 

Step 1: Identify the Cations that Give Precipitates

The cations that react with \( K_4[{Fe(CN)}_6] \) to form precipitates are:

• \( Cu^{2+} \): Forms a blue precipitate of \( Cu_2[{Fe(CN)}_6] \).
• \( Fe^{3+} \): Forms a blue precipitate of \( Fe_4[{Fe(CN)}_6]_3 \).
• \( Zn^{2+} \): Forms a white precipitate of \( Zn_2[{Fe(CN)}_6] \).

Step 2: Identify the Cations that Do Not Give Precipitates

The following cations do not form characteristic precipitates with \( K_4[{Fe(CN)}_6] \):

• \( Ba^{2+} \)
• \( Ca^{2+} \)
• \( NH_4^+ \)
• \( Mg^{2+} \)

Conclusion

The cations that form characteristic precipitates with \( K_4[{Fe(CN)}_6] \) are:

• \( Cu^{2+} \)
• \( Fe^{3+} \)
• \( Zn^{2+} \)

This gives us a total of 3 cations. Therefore, the correct answer is \( \boxed{(3)} \).